This project is concerned with the purification and characterization of human leukemia-lymphoma (HLL) associated cell membrane antigens, and with the preparation of specific anti-HLL antibody reagents for clinical use as well as for immunochemical studies of human malignant lymphoreticular cells. An effective system for isolating relatively large quantities of HLL-associated cell membrane antigens has been developed. This system consists of mechanical disruption of accumulated HLL cells, differential centrifugation to obtain cell membranes, solubilization of cell membrane antigens by deoxycholate treatment, lectin affinity chromatography, gel filtration and a novel scheme of passive immunoaffinity chromatography. HLL-associated antigen preparation obtained by utilizing this system was shown to be very useful in preparing monoclonal anti-HLL antibodies and conventional anti-HLL antisera as well as in biochemical and immunological characterization of HLLassociated cell membrane antigens. Among several anti-HLL monoclonal antibodies (Mabs) generated, a Mab, designated SN1, was most extensively studied with regard to specificity. SN1 was characterized by a sensitive microscale radioimmunoassay using a variety of cultured and uncultured human cells. In selected cases, the cell specimens were further tested by immunoperoxidase staining and an immunofluorescence staining test. Among the various cultured malignant and nonmalignant cell lines, SN1 reacted only with leukemia T-cell lines derived from patients with T-cell type acute lymphoblastic leukemia (T-ALL). In the case of uncultured cell specimens derived from cancer patients, SN1 reacted with four of four cases of T-ALL but did not react with specimens derived from 41 patients with other types of cancer. SN1 did not react with any normal human cell specimens tested, either cultured or uncultured. These specimens include normal lymphoblastoid cell lines, thymocytes, bone marrow cells, spleen cells, lymph node cells, peripheral blood mononuclear cells, lymphocytes containing B and T cells, purified T cells, monocytes, granulocytes, erythrocytes and platelets. Furthermore, SN1 did not react with phytohemagglutinin-activated T cells nor with concanavalin A-activated T cells. The results show the usefulness of SN1 in the diagnosis of cancer patients and suggest its therapeutic potential.